The present invention relates to identification card printing systems adapted to print an image, corresponding to an image file, onto card substrates. More particularly, the present invention relates to adjusting the image file to improve the contrast of the image.
Identification cards are widely used to carry information typically relating to the cardholder. The use of such identification cards is becoming more and more wide spread and they are used for many purposes, such as drivers"" licenses and identification badges. Identification card printing systems are used to print an image, which contains textual and graphical information, on card-like substrates, such as plastic cards.
Identification card printing systems can be thermal based printing systems, which use a thermal printhead to print the desired image that is formed of an array of pixels. Typical thermal printheads include a line of resistive heating elements, each of which can print one pixel of the image at a time. The resistive heating elements are uniformly deposited closely together in a single line having, for example, a resolution of 200 or 300 heaters per inch. A ribbon having primary color panels is positioned between the resistive heating elements and the substrate. When the ribbon is heated by the resistive heating elements, black, white, and colored dye or other material is presented to the substrate to form a pixel at each of the resistive heating elements. The color and gray level (intensity) of the pixels is controlled by an electric current that is selectively and controllably applied to each of the resistive heaters. The array of pixels that form the image is printed on the substrate a line at a time. Each of the lines forms several rows of the pixel array, the number of which corresponds to the number of resistive heating elements used by the printhead.
The sharpness of an image, that is the amount of detail and the maximum contrast between adjacent pixels of the image, is limited by the printhead. Identification card printing systems prefer high contrast and high definition printing to properly present photos and security marks. Unfortunately, low-cost printheads tend to be able to produce only poor contrast images. Consequently, a need exists for a method of improving the contrasts of images printed using identification card printing systems.
The present invention is directed to a method for use with an identification card printing system to improve contrast of an original image represented by an array of pixels each having a pixel level representing an intensity of the pixel. In general, a pixel of the pixel array is selected and an adjusted pixel level for the selected pixel is calculated based upon its pixel level and at least one proximal pixel level. The proximal pixel level is defined as the pixel level of a pixel located proximate to the selected pixel. Finally, the steps of selecting a pixel and calculating an adjusted pixel level for the selected pixel are repeated for substantially all of the pixels of the pixel array to form an adjusted pixel array that represents an adjusted image having greater contrast than the original image. The present invention is also directed to an identification card printing system that utilizes the above-described method to improve image contrast.